The world's principal industrial process for synthesizing ammonia are the Haber-Bosch process and similar methodologies which combine molecular nitrogen with molecular hydrogen over solid catalysts at high temperatures and elevated pressures. These processes require relatively large amounts of energy, are technically very sophisticated, and are based primarily on the use of fossil fuels as the source of hydrogen. Because of their inherent nature and demanding technical requirements, such methodologies are appropriate only for large scale industrial producers which are able to provide the requisite ingredients in the necessary volumes, offer centralized production facilities, and maintain requisite distribution systems for effective use of the process.
Alternatives to large scale industrial methods for the production of ammonia have therefore been sought with the result that considerable research has been performed to find small scale, economically viable, and less energy demanding methods. One development has been the use of metal oxide catalysts and the use of gaseous nitrogen in the air. Exemplifying this development is U.S. Pat. No. 2,500,008 which describes the synthesis of ammonia from hydrogen and nitrogen which are combined with a finely divided iron oxide catalyst and subjected subsequently to ultrasonic vibrations. Another approach involves catalytic processes which synthesize ammonia from nitrogen and water without the use of elemental hydrogen, employing instead various wavelengths of photoenergy. Consistent with this approach is the use of solar energy in various forms as the single energy source and the use of water almost exclusively as the sole reducing agent. Exemplifying this latter approach are the following developments: "Photolysis of Water and Photoreduction of Nitrogen on Titanium Dioxide", Journal Of The American Chemical Society 99:7189-7193 (1977) which describes the photoreduction of nitrogen into ammonia using titanium dioxide alone or when doped with iron, cobalt, molybdenum or nickel, or using iron oxide alone; U.S. Pat. No. 4,113,590 which describes the synthesis of ammonia and hydrazine by reduction of gaseous nitrogen with water using metal oxide catalysts under the influence of ultraviolet light; U.S. Pat. No. 4,427,510 which describes the synthesis of nitrogen-containing compounds by combining metal oxide compounds with gaseous nitrogen, a reducing agent such as water, and sunlight or artificial light; wavelengths in the visible range; and U.S. patent application Ser. No. 634,322 filed July 25, 1984 and allowed Feb. 28, 1986 which describes a method of producing ammonia with or without the use of photoenergy using a solid metal oxide catalyst and an organic compound in aqueous medium.
Essential in each of these photoassisted processes and techniques is the presence of an active catalyst without which the synthesis of ammonia would not occur. A number of useful catalysts are known and conventionally employed in this art. They include CoO, Co.sub.3 O.sub.4, Cr.sub.2 O.sub.3, .alpha.-Fe.sub.2 O.sub.3, MoO.sub.3, Nd.sub.2 O.sub.3, PbO, Pr.sub.6 O.sub.11, TeO.sub.2, WO.sub.3, La-Fe-oxide, La-Ti-oxide, Sr-Ti-oxide, Co-Mo-Ti-oxide, Pt-La-Ni-oxide, Pt-Sr-Ti-oxide, Zn-Fe-oxide, and TiO.sub.2. These conventionally known metal oxides are useful individually or in combination; and may be used without any preconditioning or, optimally, may be pretreated to increase their catalytic activity. Examples of techniques for preconditioning of catalysts are described within U.S. Pat. Nos. 4,427,510 and 4,113,590.
Of the conventionally known catalysts iron oxide, typically in the form of alpha ferric oxide [.alpha.-Fe.sub.2 O.sub.3 ] is one of the most widely known and used. .alpha.-Fe.sub.2 O.sub.3 occurs in nature as the mineral hematite. It has the corundum structure where the oxide ions form a hexagonally close-packed array with Fe(III) ions occupying octahedral interstices. .alpha.-Fe.sub.2 O.sub.3 is an n-type semiconductor; has a band gap of about 2eV.sup.1 ; and can absorb at least 40% of the solar flux at ground level. It has, accordingly, been investigated as a light-absorbing electrode for photoassisted electrolysis of water [Wilhelm et al., J. Electrochem. Soc. 126:419 (1979); Giordano et al., Int. J. Hydrogen Energy 8:763 (1983)]; as an electrode for unbiased photoelectrochemical dissociation of water [Leygraf et al., J. Phys. Chem. 86:4484 (1982); Turner et al., Chem. Phys. Lett. 105:581 (1984)]; and as a catalyst in the photoassisted reduction of nitrogen to ammonia by water or by aqueous organic fluids [Schrauzer et al., J. Am. Chem. Soc. 99:7189 (1977); Lichtin and Vijayakumar, Abstracts of the 163rd Meeting of the Electrochemical Society, May 8-13, 1983, p. 782; Lichtin et al., Proceedings of the June 1985 meeting of the International Solar Energy Society, Montreal, Canada; and U.S. Pat. No. 4,427,510].
Nevertheless, despite all these investigations and applications of .alpha.-Fe.sub.2 O.sub.3, a number of deficiencies and shortcomings remain. These include: a recognized and recurring need for an effective and efficient catalyst which will promote the reaction of pure gaseous nitrogen or air with liquid water or water vapor in combination with photoenergy to yield ammonia; a long standing desire for a catalyst which maintains activity over an extended time such as a period of at least 100 hours in duration; and a widely acknowledged need for a catalyst which is more active in promoting the photoassisted reduction of dinitrogen by water than .alpha.-Fe.sub.2 O.sub.3 is and maintains this level of activity longer than .alpha.-Fe.sub.2 O.sub.3 does. Insofar as is presently known, there is no iron oxide catalyst of any formulation which is capable of providing these distinct advantageous.